How Corals Can Overcome Ocean Acidification & Global Warming

In a paper published in Nature Climate Change, McCulloch et al. (2012) describe how biogenic calcification occurs within an extracellular calcifying fluid located in the semi-isolated space between a coral's skeleton and its calicoblastic ectoderm, where during active calcification the pH of the calcifying fluid (pHcf) is often increased relative to ambient seawater pH. At a typical seawater pH of ~8.1, for example, they state that the pH of aragonitic corals shows a species-dependent range of 8.4 to 8.7, representing a systematic increase in pHcf relative to ambient sea water (ΔpH) of ~0.3-0.6 units. In fact, they report that in situ measurements of pH within the calcifying medium of live coral polyps using microelectrodes (Al-Horani et al., 2003; Ries, 2011) and pH-sensitive dyes (Venn et al., 2011) have registered enhanced pHcf values between 0.6 and 1.2 - and sometimes up to 2 - pH units above seawater during the day, when both net production and calcification are highest.

Using a model of pH regulation combined with abiotic calcification, McCulloch et al. (2012) additionally show that "the enhanced kinetics of calcification owing to higher temperatures has the potential to counter the effects of ocean acidification," adding that "the extra energy required to up-regulate pH is minor, only <1% of that generated by photosynthesis," which highlights the importance of maintaining the zooxanthellae-coral symbiosis for sustaining calcification. And they further note, in this regard, that their model predicts "a ~15% increase in calcification rates from the Last Glacial Maximum to the late Holocene," which increase they describe as being "consistent with the expansion of tropical habitats that occurred during this time despite PCO2 increasing."

Projecting into the future with their experimentally-verified model, the four researchers assess the response of coral reefs to both global warming, with mean tropical sea surface temperatures ~ 2°C higher, and with PCO2 increasing from present-day levels to ~1,000 ppm by the year 2100." And for this scenario, they report that their model predicts "either unchanged or only minimal effects on calcification rates." Thus, from a strictly chemical and kinetic perspective, their model indicates that "ocean acidification combined with rising ocean temperatures should have only minimal effects on coral calcification," which they describe as "a direct outcome" of corals' ability to up-regulate pH at the site of calcification.

Additional References
Al-Horani, F.A., Al-Moghrabi, S.M. and de Beer, D. 2003. The mechanism of calcification and its relation to photosynthesis and respiration in the scleractinian coral Galaxea fascicularis. Marine Biology142: 419-426.